Summary of Work: We have cloned the large-conductance, calcium- and voltage-activated potassium channel (BKca) that dominates membrane excitability in rat pituitary tumor cells (GH4C1) and studied its regulation by heterologous expression of the pore-forming alpha subunit. We have identified a structural basis for inhibition of these channels by protein kinases. We have also studied the inwardly- rectifying potassium channel (Kach) through which acetylcholine regulates excitability in acutely dissociated guinea pig atrial muscle cells. We have demonstrated that ser/thr phosphatases are both necessary and sufficient for Kir regulation by acetylcholine, and we have identified novel mechanisms for Kach regulation by estrogen and by tetraethylammonium ions. - Potassium channels, protein phosphatase, pituitary, heart muscle